In recent years, efforts have been made to increase the resolution and employ more gradations for display devices typified by thin displays for providing the users with video images that are better in image equality and more realistic. The image signal that is supplied to the display devices has become digital in nature. Generally, 6-bit to 8-bit data per pixel are assigned to each of color components including R (red), G (green), and B (blue).
If the number of bits that can be displayed on a display device and the number of bits of an image signal are the same as each other, then the display device basically uses an input signal as it is to display an image. However, the number of bits that can be displayed on a display device and the number of bits of an image signal are often different from each other. If the number of bits of an image signal is greater than the number of bits that can be displayed on a display device, then the display device reduces the number of bits of the image signal according to a process of rounding off low-order bits of the image signal, a dither process, an FRC (frame rate control) process, or the like.
Conversely, if the number of bits of an image signal is smaller than the number of bits that can be displayed on a display device, then low-order bits are added to increase the number of bits of the image signal (gradation expanding process). The gradation expanding process is also employed when the data of the image signal are processed in the display device depending on the characteristics of the display device. Furthermore, the gradation expanding process is carried out in order to increase the processing accuracy even if the number of bits of the image signal and the number of bits that can be displayed on the display device are the same as each other. In this case, after the gradation expanding process is carried out, the number of bits of the image signal is converted into the number of bits that can be displayed on the display device according to the dither process, the FRC process, or the like.
The gradation expanding process is also employed for other purposes than converting the number of bits of the digital signal and increasing the processing accuracy. For example, if the number of bits of the digital signal is small, then false contours appearing as contour lines (gradations which should vary continuously in a plane are not perceived as smoothly varying, but are recognized as contour lines) become apparent in an area containing smoothly varying gradations. The gradation expanding process is employed as a technique for preventing such false contours.
Generally, the gradation expanding process is classified into two types, i.e., (1) a process of performing the same processing on all image signals, and (2) a process of extracting an image signal of a particular image and processing only necessary pixels.
(1) The process of performing the same processing on all image signals may be a first process of adding dither noise or random noise. This process is capable of slightly reducing false contours though it has a problem in that the added noise component is noticeable.
There is a second process of adding the values of high-order bits as low-order bits. For example, in order to convert a 6-bit input signal “101101” into an 8-bit input signal, the values of two high-order bits of the input signal are added as two low-order bits to convert the input signal into “10110110”. There is also a third process of simply adding 0 or 1 as a low-order bit. Though these second and third processes are simple, they cannot reduce false contours because the gradation difference is not reduced in a gradation-varying area.
(2) The process of extracting an image signal of a particular image and processing only necessary pixels may be a first process of performing low-pass filter (LPF) processing on a false-contour area, as disclosed in Japanese Patent Laid-Open No. 63-15576 (hereinafter referred to as Patent Document 1). According to this background art, in order to reduce false contours that are produced by performing gamma correction (image processing) on a digital image signal, an area where false contours are produced is adaptively judged, and an integrated value of image signals of nearby pixels in the area is output (synonymous with LPF processing). The LPF processing reduces the gradation difference in the area where false contours are produced.
However, if the intervals at which the false contours are produced (in other words, contour line intervals) are greater than the filter size (the integrated range of nearby pixels), the above process allows the viewer to easily distinguish between a filtered region and an unfiltered region in an area containing smoothly varying gradations, and hence does not lead to an appreciably improved image quality even though the false contours are reduced.
According to a second process disclosed in Japanese Patent Laid-Open No. 4-165874 (hereinafter referred to as Patent Document 2), in order to reduce false contours that are produced by performing gamma correction (image processing), when an area containing smoothly varying gradations (smooth gradation area) is determined, the gradation values of pixels between contour lines of the false contours in the area are found by linearly interpolating the gradation values of pixels on contour lines. This process goes not suffer from the problems of the first process because uniform gradation changes are achieved in the smooth gradation area.
It follows from the foregoing that the gradation expanding process for detecting particular information of pixels and performing linear interpolation depending on the detected result is considered to be preferable from the standpoint of the reduction of false contours. The process using linear interpolation is also disclosed in Japanese Patent Laid-Open No. 2000-304400 (hereinafter referred to as Patent Document 3), Japanese Patent Laid-Open No. 2003-333348 (hereinafter referred to as Patent Document 4), and Japanese Patent Laid-Open No. 2004-54210 (hereinafter referred to as Patent Document 5).
According to Patent Documents 3 through 5, unlike Patent Document 1 and Patent Document 2, the gradation expanding process is carried out in order to solve the problem of false contours that are produced because the bit depth of a digital image signal is small or in order to maximize the gradation capability of a display device. However, the process of linear interpolation is the same as with Patent Document 1 and Patent Document 2.
Patent Document 3 discloses an image processing device comprising a false contour detector and a pixel value converter. The false contour detector detects a false contour on the conditions that after the same luminance level continues horizontally over two pixels or more, the luminance level increases by 1 (condition 1), and after the luminance level decreases by 1, the same luminance level continues horizontally over two pixels or more (condition 2). The pixel value converter performs linear interpolation on the detected false contour.
Patent Document 4 discloses a color signal expanding device comprising a data distribution detector and a data depth expander. The data distribution detector extracts (detects) an area where colors vary gradually from the distribution of gradation data. Specifically, the data distribution detector detects an area where, in a pixel group K of the same gradation, the number of pixels is equal to or greater than a lower limit threshold P and equal to or smaller than an upper limit threshold Q and the gradation difference with the pixels of an adjacent pixel group is equal to or smaller than a decision threshold S. Then, the data distribution detector performs linear interpolation on the area where colors vary gradually to determine a gradation value to be added to the area. The data depth expander adds the gradation value to an expanded area and generates expanded image data representing an expanded data depth of the color signal.
Patent Document 5 discloses an image processing device comprising a detecting means and a signal expanding means. The detecting means determines whether there is a pseudo—(false) contour or not by determining whether or not the difference between a first position where the same pixel data continues and a first position where next pixel data continues is equal to a width over which the same data continues, and also determining whether the gradation value of an area where the same pixel data continues is greater or smaller by 1 than the gradation value of an area where the next pixel data continues. In order to obtain a smoothly continuous image in the area where the pseudo-contour is produced, the signal expanding means expands the gradation of the image data smoothly and linearly (by performing linear interpolation).
The arrangements disclosed in Patent Documents 3 through 5 pose no problem in that the gradation expanding process is performed according to the linear interpolation disclosed in Patent Document 2. However, they are problematic in that they are susceptible to noise components, are unable to detect an area appropriately, and fail to perform the gradation expanding process as desired because of the conditions used when detecting a linear-interpolation-applicable area for performing the gradation expanding process by way of linear interpolation, i.e., the conditions that an area where the same gradation continues should be positioned adjacent to the linear-interpolation-applicable area and the gradation difference in the area is equal to or smaller than a constant value.